This application claims the priority of German patent application DE 199 42 746.1, filed in Germany on Sep. 8, 1999 and German patent application DE 100 02 771.7, filed in Germany on Jan. 22, 2000, the disclosures of which are expressly incorporated by reference herein.
The invention relates to an internal combustion engine having a variable valve timing. It is known that a charging cycle has two decisive requirements. On the one hand, by means of an exchange, the working gas is brought to the starting condition of the cyclic process and, on the other hand, the oxygen required for the fuel combustion is provided in the form of fresh air. The opening and closing of the valves for the charge cycle takes place by means of the camshaft. Pins or pushrods are normally engaged with a camshaft for this purpose. The pushrods are used to press out the used-up gas and take in the fresh gases by operating to press the separately designed charge cycle valves against the valve springs. Thus, shortly before the upper dead center, the intake valve opens up while the outlet valve is still open. The outlet valve closes shorter after the upper dead center and, while the intake valve is open, the piston, which moves downward, can take in fresh air. The time, during which both valves (intake and outlet valve) are both open simultaneously is called the overlapping time or overlapping surface. The overlapping time and the size of the valve lifts are significant for the provided torque and are therefore responsible for the engine output and thus also for the driving comfort. The switching thresholds for the continuous change of the valve timing periods and of the valve lift are determined as a function of the rotational speed and of the load applied to the internal combustion engine and are stored in a corresponding characteristic rotational speedxe2x80x94load diagram. The variable valve timing per se is a rather complicated solution of spatial cam profiles on a longitudinally displaceable camshaft but permits a clear torque gain in the entire rotational speed range of the engine.
From German Patent Document DE 198 14 888 A1 (corresponding to U.S. Pat. No. 5,992,361), a variable valve timing mechanism is known, in the case of which the characteristic valve lift curve is changed in order to achieve an optimal engine output power. The valve timing mechanism is arranged on the side of the intake valve and/or the outlet valve. Here, at the start of a change of the operating condition of the internal combustion engine from a low rotational engine speed to a high rotational engine speed, the valve opening duration of the intake valve and/or of the outlet valve is changed such that the overlapping duration of the intake valve and the outlet valve is increased. As a result, it is possible to ensure a uniform course of the torque curve also during the transition from the low rotational engine speed to the high rotational engine speed and to avoid step-type sections in the course of the torques which reduce the driving comfort. The acceleration therefore takes place in a steady manner.
An object of the present invention is to provide improvements in the engine operation as compared to the prior art arrangements.
At least this object is achieved by preferred embodiments of the invention by providing an arrangement where the switching thresholds are weighted in response to respective engaged transmission gears and/or respective engine operating termperatures.
As a result of a weighting of the switching thresholds according to the invention for a change of the valve timing periods and of the valve lift by the respective engaged gear and/or the engine temperature, it is achieved that a small valve lift can be used, as desired, into high load and rotational speed ranges, which has a positive effect on the fuel consumption. Simultaneously, when switching from a small to a large valve lift, no torque jump will occur and a jolt-free operation of the internal combustion engine is therefore possible.
A known switching threshold for a switch-over of the valve lift is defined as a function of the load and the rotational engine speed. For reasons of fuel consumption, it is endeavored to operate the internal combustion engine in a largest possible operating range with a small valve lift. Advantageously, the defining of the switching thresholds is determined in the application or on an engine test bed and is then stored as corresponding values in the characteristic rotational speed-load diagram in the control unit. By means of the suggested solution, it is achieved that, also at high speeds, the operation can take place at a small valve lift which is favorable with respect to the fuel consumption. During the switching-back, the valve lift switch-over does not take place to the same value in the characteristic rotational speedxe2x80x94load diagram but offset by a selectable or definable difference. Thus, the curve of the valve switching has the shape of a hysteresis which, in addition is dependent on the gear, whereby the driving comfort is improved further.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.